Ethylene elastomer compositions typically contain ethylene, an alpha-olefin such as propylene, and a third component, such as a diene, to facilitate vulcanization. The choice of a third component can greatly influence the nature of branching reactions and resultant molecular architecture of the polymer. For example, when ethylidene norbornene (ENB) is used as the diene, as is commonly done, long chain branching through the diene is facilitated through cataionic coupling reactions in Ziegler-Natta (Z-N) polymerization, while linear polymers are produced with metallocene catalysts.
EPDMs containing VNB, which is a non-conjugated diene, are known from EP843698; EP843702 and EP843701. These polymers typically have a high level of long chain branching (LCB) when synthesized using Ziegler-Natta catalysts. High levels of LCB improve processability, but may impair physical properties of the final product after conversion of the polymer by extrusion or molding. Other methods of EPDM production are described in WO99/00434, WO 99/45062, U.S. Pat. No. 6,806,336, U.S. Pat. No. 6,096,849, and U.S. Pat. No. 6,319,998.
VNB-derived EPDM provides improved cure rate and performance in free-radical curing, improved processability from the highly branched structure, and requires a low level of diene to provide suitable physical properties in the final product comparable to ENB-derived EPDM.
The lack of branching in metallocene polymerization of ENB is believed to be due to the inability of the pendent double bond to incorporate into the polymer backbone due to steric effects. When the same metallocene catalyst is used to polymerize other third components such as divinyl benzene (DVB) or VNB, the pendant double bond incorporation is facile and leads to copious amounts of branching and gel. When used as the third component, such monomers produce a polymer with very desirable vulcanization characteristics in addition to cross-linking such as with peroxides, when compared to ENB. Therefore, it would be beneficial to the rubber industry to devise a method of incorporating dienes, particularly VNB, without excessive long chain branching or gel formation when a metallocene catalyst is employed.